# Electrical Engineering Electric Machines And Power Fund

AUK (Spring 2021) ELEG 310 Mid-Term Exam

(**)

Dr. Ashraf Zaher 3/13

Q1. For the magnetic circuit, shown in Fig. (1-a), i1 = 0.8 A and i2 = 1.2 A, and r is 1800,

which is assumed to be constant for the core material. The fringing effect is 15%. Find:

i. the total reluctance of the magnetic circuit, ℜT,

ii. the total flux, φT, and indicate its direction (CW or CCW),

iii. the magnetic field density of the air gap, Bg, and

iv. the value of the current I, in Fig. (1-b) that will produce the same value of the

magnetic field intensity, Hc, of the core in Fig. (1-a).

[6+6+3+5 marks]

(a)

(b)

Figure (1)

Boom arts

Tha

Magoon

AUK (Spring 2021) ELEG 310 Mid-Term Exam

(**)

Dr. Ashraf Zaher 5/13

Q2. A 22 KVA, 1100V/220V single phase transformer has the following parameters:

• Rp = 0.4 Ω and Rs = 0.08 Ω

• Xp = 0.5 Ω and Xs = 0.1 Ω

• Rc = 120 KΩ and Xm = 12 KΩ

i. Draw the equivalent circuit of the transformer referred to the secondary side, while

showing all numerical values,

ii. Calculate the voltage regulation, VR, at the full-load condition, assuming the power

factor to be 0.9 lagging,

iii. Draw the phasor diagram for the condition of part (ii),

iv. Find the efficiency, η, for the condition of part (ii), and

v. Calculate the voltage stepping up ratio and the new apparent power rating, if this

transformer is connected as an auto transformer.

[4+4+4+4+4 marks]

Kaplan a

Stag

AUK (Spring 2021) ELEG 310 Mid-Term Exam

(**)

Dr. Ashraf Zaher 7/13

Q3. A 400 V, 50 Hz, 8-pole, Y-connected synchronous generator has a full load armature

current of 40 A at a 0.85 PF leading. The generator has a per phase synchronous

reactance of 1.25 Ω, and a negligible synchronous resistance. Friction and windage

losses are 1.0 KW, while the core losses are 0.5 KW, at full load. The field current has

been adjusted to generate a terminal voltage of 400 V at no load. Find:

i. the magnitude of the internal voltage, EA, and use it to calculate the terminal

voltage, when the generator is loaded with the rated current at a 0.85 PF leading,

ii. the phasor diagram of the generator,

iii. the active power and the reactive power (supplied/delivered) by the generator,

iv. the voltage regulation, VR,

v. the efficiency, η, and

vi. the applied torque through the prime mover.

[5+4+2+3+3+3 marks]

BEN

Troodos

own